There is significant economic incentive to convert Fischer-Tropsch (F-T) wax to high quality lube basestocks, especially base oils with properties and performance comparable to, or better than, those of polyalphaolefins (PAO). The upgrading of Fischer-Tropsch wax greatly relies on advanced wax isomerization technology that transforms linear paraffins to multi-branched isoparaffins with minimal cracking.
Processes for converting Fischer-Tropsch wax to paraffinic lube base-stocks are known. A typical process is a two-stage process that hydroisomerizes Fischer-Tropsch wax to a waxy isoparaffins mixture in the first step, followed by either solvent dewaxing or catalytic dewaxing the waxy isoparaffins mixture in the second step to remove residual wax and achieve a target lube pour point.
The hydroisomerization catalysts disclosed previously, such as Pt supported on amorphous aluminosilicate or Zeolite Beta (Beta), normally possess large pores that allow the formation of branch structures during paraffin isomerization. Examples of other large pore molecular sieves include ZSM-3, ZSM-12, ZSM-20, MCM-37, MCM-68, ECR-5, SAPO-5, SAPO-37 and USY. However, these large pore catalysts are not selective enough to preferentially convert normal and lightly branched paraffin waxes in the presence of multi-branched isoparaffin molecules. As a result, the isoparaffin products derived from Fischer-Tropsch wax often contain residual wax that needs to be dewaxed in order to meet target lube cloud points or pour points. The cloud point of a lube is the temperature at which the first trace of wax stalls to separate, causing the lube to become turbid or cloudy (e.g., ASTM D2500). The pour point of a lube is the temperature at which lube and wax crystallize together as a whole and will not flow when poured (e.g., ASTM D97). Dewaxing can be achieved by additionally using either a solvent dewaxing process or a catalytic dewaxing process.
Most selective dewaxing catalysts used in a catalytic dewaxing process have relatively small pore structures and catalyze lube pour point reduction by selectively cracking normal and lightly branched paraffin waxes. Such dewaxing catalysts usually have low paraffin isomerization selectivity.
Few catalysts have been reported to be efficient in catalyzing both hydroisomerization and dewaxing of paraffin wax to low pour point lubes. One example of such catalysts is a noble metal, such as Pt, supported on SAPO-11. It was previously assumed that oval-shaped pore structures are common feature of isomerization and dewaxing catalysts. See, for example U.S. Pat. No. 5,246,566.
There remains a need therefore and a higher isomerization selectivity to achieve a low enough pour point with minimal molecular weight changes.